String motion sensors, commonly known as pickups, are installed on guitars, bass guitars, mandolins and other stringed musical instruments to convert the sound produced by the vibrating strings to an electronic signal. In various applications, the electronic signal generated by a pickup may be modified using analog and digital signal processing techniques, amplified, and recorded on a suitable sound recording medium before being converted back to a sound signal by a speaker or other output transducer. Conventional musical instrument pickups use different physical principles, including variations in magnetic reluctance, the Hall effect, and the piezoelectric effect, to detect the motion of ferromagnetic strings.
Magnetic reluctance pickups typically comprise one or more ferromagnetic pole pieces, a magnetic source and a coil with output terminals that surrounds the pole pieces. When the pickup is positioned near the ferromagnetic strings of a musical instrument the magnetic source, pole pieces and strings may be modeled as a magnetic circuit with a magnetic flux in each of the elements. The magnetic flux in a pole piece is partially dependent on the distance between the string-sensing surface of the pole piece and a string. String vibrations change the pole-to-string distance and the pole piece flux. The coil surrounding the pole pieces is said to link the flux in the pole pieces and an electromotive force is developed in the coil when the magnetic flux changes. An electronic signal is developed at the output terminals of the coil in response to the electromotive force.
The frequency-dependent response function of a magnetic musical pickup is nonlinear and the input string-motion signal is distorted by the pickup in the process of converting it to an electronic signal. This distortion imparts certain tonal attributes to the string-sensing process and, when properly controlled, adds desirable and highly musical qualities to the output signal.
Magnetic reluctance pickups came into common usage during the 1950's when hard ferromagnetic material and sensor technologies evolved to a point that the pickups could be economically mounted on a musical instrument. Magnetic pickups have been developed for many different instruments and a significant commercial market exists for magnetic guitar pickups. For purposes of clarity, the features of the present invention will be discussed with reference to a 6-string guitar with ferromagnetic strings. It will, however, be obvious to those skilled in the art that the scope of the invention is not limited to 6-string guitars and magnetic pickups that embody features of the invention may be mounted on many different instruments. Other instruments that are commonly equipped with magnetic pickups include, but are not limited to, 12-string guitars, bass guitars, mandolins, and steel guitars.
Magnetic musical instrument pickups may be classified into broad categories that reflect differences in basic design and tonal quality. Pickups in the ‘single coil’ category have key design features that are shared by the pickups disclosed in U.S. Pat. No. 2,612,072 issued to H. de Armond on Sep. 30, 1952, U.S. Pat. No. 2,573,254, No. 2,817,261, No. 3,236,930, and No. 4,220,069 respectively issued to Leo Fender on Oct. 30, 1951, Dec. 24, 1957, Feb. 22, 1966, and Sep. 2, 1980 and U.S. Pat. No. 2,911,871 issued to C. F. Schultz on Nov. 10, 1959. The ‘single coil’ name derives from the fact that pickups in this category comprise a set of string-sensing ferromagnetic pole pieces with a magnetic flux that is linked by a single, string-sensing coil of wire. Some single coil pickups have pole pieces that are formed from magnetized hard ferromagnetic materials that generate the flux in the pickup. In other single coil designs a separate permanent magnet induces magnetic fields in the pole pieces. Single coil pickups have no means for external noise rejection and are sensitive to external electromagnetic noise sources.
The external noise sensitivity of a magnetic pickup may be significantly reduced by adding a second wire coil to the pickup. The second coil is designed to generate an electronic output signal at its terminals with a noise component that is similar to the noise output of the first coil. Noise reduction is accomplished by connecting the two coils so that the noise signals have opposite phases.
Noise-reducing humbucking pickups or ‘humbuckers’ share key design features with the devices that are disclosed in U.S. Pat. No. 2,896,491 issued to Seth Lover in Jul. 28, 1959 and U.S. Pat. No. 2,892,371 issued to J. R. Butts on Jun. 30, 1959. Pickups in this class have at least two-string sensing coils, each linking a magnetic flux in a separate set of string-sensing pole pieces. The magnetic field direction in the poles and the direction of signal propagation within the coils are selected so that a large portion of the string-generated signals from the two coils have an in-phase, additive relationship and a large percentage of the common-mode noise signals from the two coils have an out-of-phase, subtractive relationship. Split-blade designs such as the Lindy Fralin Split-blade pickups manufactured by Lindy Fralin of Richmond, Va. also fall into the ‘humbucking’ category. In most cases, the output signal amplitude of a humbucking pickup is greater than that obtained from a single coil pickup and the output noise signal is smaller.
Noise-cancelling single coil pickups have tonal characteristics similar to those of single coil pickups and comprise a single set of string-sensing poles, a string-sensing coil and a noise-cancelling coil that is connected to the string-sensing coil. In some designs, the noise-cancelling coil links the flux in a set of passive pole pieces. Illustrative noise-cancelling single coil pickups are disclosed in U.S. Pat. No. 7,166,793 issued to Kevin Beller on Jan. 23, 2007, U.S. Pat. No. 7,189,916 issued to Christopher I. Kinman on Mar. 13, 2007, and U.S. Pat. No. 7,227,076 issued to Willi L. Stich on Jun. 5, 2007.
Active circuitry is incorporated into some magnetic pickups to decrease the output impedance of the pickup, increase the output amplitude and, in some cases, modify the pickup tone. Active pickups with different coil and pole piece designs are manufactured, for example, by EMG, Inc. of Santa Rosa, Calif.
The design and manufacture of magnetic musical instrument pickups are described from an historical and lay engineering perspective in The Guitar Pickup Handbook, the Start of Your Sound by Duncan Hunter (Backbeat/Hal Leonard, New York, 2008), Pickups, Windings and Magnets and the Guitar Became Electric by Mario Milan (Centerstream, Anaheim Hills, 2007) and Electric Guitar, Sound Secrets and Technology by Helmuth Lemme (Elektor, Netherlands, 2012). On a more technical level, Engineering the Guitar, Theory and Practice by Richard Mark French (Springer, New York, 2009) contains a chapter on Guitar Electronics and a thorough treatment of musical sound quality and tone as viewed from an engineering and physics perspective.